Learn French: Les chiens sont lents.

Yes! While the singular word for "the" changes based on gender (le for masculine, la for feminine), the plural form is always les, no matter what gender the noun is. Here, chiens is masculine plural, but it would still be les for a feminine plural noun.

Do I pronounce the "s" at the end of les?

Not in this sentence. Because the next word (chiens) starts with a consonant, the "s" in les is silent. It sounds roughly like "lay". You only pronounce that "s" (making a "z" sound) if the following word starts with a vowel or a silent h.

Why does the word lents have an "s" at the end here?

In French, adjectives like lent (slow) must match the noun they describe in both gender and amount. Since les chiens is plural, we add an "s" to the adjective to make it plural too.

There are a lot of "s" and "t" endings in this sentence. Are they all silent?

Yes, they are! The final "s" in les, chiens, and lents, as well as the final "t" in sont and lents, are completely silent. If you read it out loud, you only hear the core sounds of the words without their final letters.